MEF2C Haploinsufficiency Syndrome (MHS) is a debilitating neurodevelopmental disorder related to autism spectrum disorders (ASD). Core symptoms of social/communication deficits and repetitive/fixed behaviors define ASD. MHS is an ASD-related syndrome characterized by ASD symptoms, such as poor reciprocal social behavior, lack of speech, and repetitive behaviors. In addition to ASD symptoms, intellectual disability, seizures, and motor movement issues are also observed in MHS patients. MHS is genetically linked to mutations and deletions in one of the two copies of the MEF2C gene on chromosome 5, termed MEF2C haploinsufficiency. There are currently no FDA approved treatments for MHS, and there is a need for more knowledge of the mechanisms behind this syndrome. Mouse models to study this syndrome have been developed, in which one copy of the Mef2c gene has a deletion in exon 2 (Mef2C+/- mice), termed Mef2c heterozygosity. These mice exhibit behaviors reminiscent of MHS, including social interaction and communication deficits. Most studies into MHS have focused on neurons in the brain but, not microglia, which also express MEF2C. In addition to clearing brain debris, microglia, the resident immune cells of the brain, have been observed shaping neural and synaptic circuitry. Since microglia also express MEF2C and have pivotal functions in the brain, the goal of this proposed fellowship is to investigate the possible contributions that microglia could have on MEF2C Haploinsufficiency Syndrome-related phenotypes. We hypothesize that Mef2c heterozygosity leads to changes in microglial cell physiology, which results in aberrant behavioral phenotypes. I plan to use mouse models through two aims to examine this hypothesis. Specific aim 1 plans to determine the role of Mef2c in microglial activation and microglial gene expression. Aim 1 proposes to study this activation in mice that are heterozygous for Mef2c in a restricted population, which includes microglia in the brain. In addition to confirming possible microglial activation, aim 1 proposes to study possible functional changes in Mef2C+/- microglia through RNA-sequencing and subsequent gene ontology analysis. Specific aim 2 proposes to characterize MEF2C Haploinsufficiency Syndrome-related behaviors in mice heterozygous for Mef2c in microglia. Expected results include findings that microglia in these mouse models have increased activation and changes in microglial function. If behavior deficits are seen in the microglia-restricted Mef2c mice, this could explain a cell type-specific neuroimmune mechanism of MHS. This proposed fellowship would assist the investigator in achieving her goal to become a physician-scientist. The background and training potential of the investigator is outlined in this fellowship application. This proposal is uniquely suited to the training needs of this investigator, including training in experimental design, technical laboratory skills, scholarship, and ethics. This research and training will be performed at the Medical University of South Carolina, which has a collaborative environment and many facilities to support the investigator?s training.